Pneumatic Impact Treatment

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The English term Pneumatic Impact Treatment ( PIT ) describes an aftertreatment process, whereby an increase in fatigue strength is achieved by hammering the surface at higher frequencies . In particular, the method is used for welded connections , whereby a significant increase in service life can be achieved. The mode of action is based on a reduction in the geometric notch effect at the seam transition, a build-up of residual compressive stresses and a solidification of the material in the post-treated area. Due to the ease of use and high reproducibility, the process is particularly suitable for industrial applications in plant, container, machine or steel construction.

Hand and control device

Fatigue strength

Under fatigue damage to or failure of components and materials is understood under cyclic loading. Cracks are formed, preferably at notches, imperfections or highly stressed zones, which continuously enlarge and ultimately lead to failure or failure of the component. In general, the greater the static strength of a material , the greater the fatigue strength . In the case of geometric irregularities such as notches, which can arise due to construction or welded connections, this relationship is only partially fulfilled due to the increasing notch sensitivity of high-strength materials. For this reason, according to current guidelines and recommendations , the fatigue behavior of welded steel connections is generally independent of the yield strength of the base material used. Due to the constantly growing demands for increased lightweight construction and an increase in the service life of components and structures, however, an improvement in the fatigue strength behavior is essential. This can be achieved, for example, by using a PIT aftertreatment for higher and high-strength, but also low-strength base materials.

Process description

The PIT process is a pneumatically operated, high-frequency hammering process, which was developed for the mechanical post-treatment of welded connections and highly stressed unwelded zones of a component. Both the processing frequency and the impact intensity can be set independently of each other, which makes it possible to meet the different requirements of different materials and weld seam geometries. A pneumatic muscle in the device converts the pressure energy into mechanical impulses, which are transmitted to the surface to be treated by one or more hardened steel bolts. In order to keep the vibrations as low as possible during the treatment, a further spring system is included so that the hand-held device is completely decoupled from the impact force. This causes a low hand arm vibration at a height of around 5 m / sec for the operator and, in addition, there is an almost constant impact force, which ensures a high level of reproducibility . The feed speed for steel is about 20 cm / min with a freely selectable machining frequency of the steel bolt or bolts of up to 80 to 120 Hz. The impact intensity can be continuously adjusted via the compressed air, whereby, in contrast to other methods, the device already operates at a pressure of is functional below 4 to 5 bar and thus has a low air consumption of around 175 to 250 l / min. Discharging the exhaust air forward to the processing point has the advantage that paint particles, metal shavings and other impurities are blown away and are not unintentionally pressed into the workpiece surface, and the flowing air cools the stud or studs, thereby significantly increasing the service life.

Mode of action

Compared to other post-treatment processes, such as grinding , shot peening or stress- relieving annealing , where an increase in fatigue strength or service life is usually only achieved through one effect, the PIT process combines the following modes of action:

Reduction of the geometric notch effect

As a result of the post-treatment, the transition from the base material to the weld seam, which is critical for the fatigue stress, is rounded off, which leads to a significant reduction in the geometric notch effect in this area. Especially in the case of sharp geometries, such as weld seam ends, this effect contributes significantly to the mode of operation.

Solidification of the material

Due to the deformation of the material, local hardening takes place in the post-treated area . Depending on the material and hardening behavior, this can lead to a significant increase in hardness and thus also to an increase in strength.

Build-up of residual compressive stresses

In addition to the local hardening, internal compressive stresses are introduced, which counteract the tensile stresses relevant to fatigue and thereby reduce the overall stress in the most heavily loaded zone. To verify the residual stress state that develops through the aftertreatment, measurements of the residual stresses using X-ray diffractometry or borehole methods can be used, but an estimation of the local residual stress state based on a numerical simulation is also possible.

research results

As part of the International Institute of Welding (IIW), the effect of weld seam treatments on fatigue strength has already been extensively investigated, from which international recommendations and application guidelines have emerged. The higher-frequency hammering was introduced under the English term High Frequency Mechanical Impact (HFMI) Treatment and based on current research results, proposals for an increase in fatigue strength dependent on the strength of the base material were developed, which are currently being added to the guidelines. Numerous test results for welded steel connections with a yield strength of 235 to 1300 MPa show, for example, that a PIT post-treatment can achieve a significant increase in fatigue strength of up to 250% in the long-term strength range (from around one million load cycles). Comparative tests on base material samples also make it clear that when the method is used, the fatigue strength of the base material can be used almost completely, which means that there is a high potential for lightweight construction for welded structures. It is also shown that this post-treatment technique is an effective way of improving existing structures.

quality control

High reproducibility and quality assurance measures are essential aspects for industrial use. In addition to a well-founded training and sensitization of the operator, the quality can be determined on the one hand by optical controls of the treatment track after application, as well as by checking the intensity of the treatment with the help of the PIT-ALMEN intensity test before the follow-up treatment. These measures ensure the consistent quality and effectiveness of the PIT process in the long term.

Advantages of PIT

The main advantage of the PIT treatment is an increase in fatigue strength and, as a consequence, the service life of welded connections and highly stressed components. On the one hand, the method allows the strength of low, higher and high-strength base materials to be used almost entirely, which results in considerable lightweight construction potential and savings in fuel consumption and pollutant emissions, but it is also possible to effectively upgrade components in operation. The compact and extremely portable PIT system also enables problem-free treatment of welds on construction sites. Due to the special mechanical structure, it is possible to integrate the system into an automated manufacturing process, for example as a structure on a robot, which can be used efficiently for large quantities and long weld seams. In general, the PIT process offers the following advantages, which have been confirmed by numerous studies in science and industry:

Application examples

The PIT process is already widely used in the industrial sector, with selected application examples being presented below.

Plant and steel construction: Structures and hydraulic steel construction

Particularly in the field of construction, a fatigue-proof construction and design is essential for safe operation over the entire service life. Based on the positive experience gained with the construction of new road bridges, the PIT process was also used for the post-treatment of welded carriageway crossing structures made of structural steel. Accompanying tests on structural components confirm the effective effect of the methodology and form the basis for subsequent application approval.

Use of PIT in steel construction

Further investigations on reference samples with structural details of hydraulic steelwork show that the technology is also particularly effective for the maintenance of previously damaged components and can be used in this area for measures to extend the service life.

Container construction: nuclear fusion reactor

To control the distortion when welding pipe sockets into the existing tank structure of a reactor, the heat input during the welding process must be kept as low as possible, which means that additional cooling with dry ice is necessary. With a PIT post-treatment of each welding layer, the residual tensile stresses caused by the joining process can be kept small, enabling practically distortion-free welding and minimizing the effort involved in subsequent processing steps.

Mechanical engineering: lightweight construction potential

In the field of mechanical engineering, the process is often used in combination with low, high and high strength steels to increase the lightweight construction potential. In the case of cranes and crane-like structures, for example, this procedure enables a substantial weight saving to be achieved while maintaining the same crane range, as a result of which operation that reduces fuel and pollutants is possible with a longer service life.

Individual evidence

  1. ↑ Radaj D., Vormwald M .: fatigue resistance, 3rd edition, Springer Verlag., 2007
  2. ↑ Research Board of Mechanical Engineering: Computational proof of strength for machine components made of steel, cast iron and aluminum materials, 6th edition. Frankfurt am Main, VDMA-Verlag, 2012.
  3. ↑ a b Hobbacher A .: IIW Recommendations for Fatigue Design of Welded Joints and Components, WRC Bulletin 520, The Welding Research Council, New York, in 2009.
  4. ^ A b Haagensen PJ, Maddox SJ: IIW Recommendations On Methods for Improving the Fatigue Strength of Welded Joints, Woodhead Publishing, 2013.
  5. ↑ Gerster P .: Increasing the service life or fatigue strength through post-weld treatment, der Praktiker, Vol. 9, pp. 302-310, 2009.
  6. ↑ Yildirim H., Marquis G .: Overview of fatigue data for high frequency treated welded joints, Welding in the World, vol. 56, pp. 82-96, 2012.
  7. ↑ Marquis G., Mikkola E., Yildirim H., Barsoum Z .: Fatigue strength improvement of steel structures by high-frequency mechanical impact: proposed fatigue assessment guidelines, Welding in the World, vol. 57, pp. 803-822, 2013.
  8. ↑ Marquis G., Barsoum Z .: Fatigue strength improvement of steel structures by high-frequency mechanical impact: proposed procedures and quality assurance guidelines, Welding in the World, vol. 58, pp. 19-28, 2014.
  9. ↑ Leitner M., Stoschka M., Eichlseder W .: Fatigue enhancement of thin-walled high-strength steel joints by high frequency mechanical impact treatment, Welding in the World, Vol. 58, No. 1, pp. 29-39, 2014.
  10. ^ Yildirim H., Marquis G .: Overview of Fatigue Data for High Frequency Treated Welded Joints, IIW-Document XIII-2362r1-11, 2011.
  11. ↑ a b Berg J., Stranghöner N .: Fatigue behavior of HFH post-treated notch details in mobile crane construction, Stahlbau 83, Heft 8, 2014.
  12. ↑ Gerster P .: Proven in practice: Maintenance and repair of welded constructions by means of higher frequency hammering, der Praktiker, Vol. 9, pp. 336-339, 2010.
  13. ↑ Gerster P., Schäfers F., Leitner M .: Pneumatic Impact Treatment (PIT) - Application and Quality Assurance, IIW-document XIII-WG2-138-13, 2013.
  14. ↑ Gerster P., Schäfers F .: Method for increasing the service life or fatigue life of components, Stahlbau 83, Issue 8, 2014.
  15. ↑ Leitner M., Stoschka M., Fössl T., Eichlseder W .: Fatigue strength of high-strength steels on welded structures, welding and testing technology, No. 1/2012, pp. 12-17, 2012.
  16. ↑ Schäfers F .: High residual compressive stresses reduce system failures, Maschinenmarkt 22, pp. 56-59, 2011.
  17. ↑ Gerster P., Leitner M., Stoschka M .: Practical Applications of a Higher Frequency Hammering Process (PIT) in Industry, Proceedings of the Join-Ex Congress, Vienna / Austria, pp. 101-112, 2012.
  18. ↑ Stranghöner N., Berg J., Butz C .: Increasing the service life of road crossings with the help of higher-frequency hammering, 17th DASt Colloquium, German Committee for Steel Construction, Weimar / Germany, pp. 109-113, 2010.
  19. ↑ Gabrys U .: Recommendations for the new construction and repair of hydraulic steel structures, proceedings for the Great Welding Conference 2011, DVS reports, pp. 61-66, 2011.
  20. ↑ Schäfers F .: Anti Aging with the Hammer, specialist journal Instandsetzung, 2011.